This study was designed to determine the frequency of hepatitis C virus (HCV) infection in Clinical Center patients who received implicated lots of Baxter Gammagard Immune Globulin Intravenous (IVIG) (Human) since September 1992. This product was voluntarily recalled by the manufacturer in February 1994 after a possible association between HCV transmission and Gammagard was identified. This study's purposes were to assess whether HCV was transmitted by Gammagard and to provide education and counseling to Clinical Center patients who might have acquired HCV infection. Patients who may have received this product were identified by several mechanisms: (1) a computerized search of patient records for the order code; (2) a review of Pharmacy IV cards; and (3) responses to a memo, sent to credentialed physicians in March 1994 requesting help in identifying patients. Many identified patients had immunodeficiencies; many were children. Participating patients had a blood sample drawn to test for biochemical and serologic markers of HCV infection: alanine aminotransferase, antibody to HCV, and polymerase chain reaction (PCR) for HCV RNA. Patients were also asked to complete a brief questionnaire regarding risk factors for HCV infection and any clinical manifestations of hepatitis. If evidence suggested HCV infection, we attempted to obtain pretreatment evaluations by asking the patients' attending physicians if stored sera predating the receipt of Gammagard were available for retrospective testing. Enrollment and testing was completed on 132 patients: 13 were identified with evidence of HCV infection (PCR positive for HCV RNA). Eleven of the 13 were found to be infected before receiving the implicated IVIG, and all 13 have a history of receiving blood or blood products. Two HCV-infected patients did not have contemporaneous stored samples preceding IVIG infusion available for testing. One of these patients had received blood years earlier from a donor who was later found to be infected with HCV; this patient had clinical and biochemical evidence of posttransfusion hepatitis years before receiving the implicated IVIG. The other patient received a single Gammagard infusion at the National Institutes of Health, and the implicated IVIG cannot be ruled out.